Among various types of stratified charge or stratified combustion engines are those of ignition chamber or prechamber design wherein a small prechamber is fed a rich mixture that is subsequently ignited and a connecting main chamber is supplied with air or very lean mixture. The stratified combustion process in such engines may be considered as instantaneous reaction since, upon ignition, the products of rich combustion issuing from the prechamber immediately mix and react with unreacted constituents of the main chamber.
The consequence of this sequence of reactions in terms of formation of nitrogen oxides (NO.sub.x) is not optimum since the products in the main chamber always pass through the slightly lean air-to-fuel ratio (approximately 16:1) in which maximum NO.sub.x is produced. Further, the effect of injecting the burning charge of rich mixture into the cold main body of lean mixture or air is to cool or quench the combustion reaction so that the combustion event and the time occupied in burning in the region of maximum NO.sub.x are extended, increasing the total NO.sub.x formation. An example of the air/fuel ratios present in the main chamber of an engine of this type during combustion is shown in FIG. 1 wherein the ratios begin at infinity and are subsequently enriched during combustion to approximately a stoichiometric air/fuel ratio of 14.7:1.